1. Field of the Invention
The present invention relates to an inkjet ink system comprising a liquid vehicle, a colorant, and a gelling agent. Methods of printing using this inkjet ink system are also disclosed.
2. Description of the Related Art
An inkjet ink composition generally consists of a vehicle, which functions as a carrier, and a colorant such as a dye or pigment. Additives and/or cosolvents can also be incorporated in order to adjust the inkjet ink to attain the desired overall performance properties.
In general, pigments alone are not readily dispersible in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions which can be used in inkjet printing. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants include water-soluble polymers and surfactants. Typically, these polymeric dispersants have a molecular weight less than 20,000 in order to maintain solubility and therefore pigment stability.
Recently, modified pigments have also been developed which provide ink compositions with improved properties, such as dispersibility, without the need for an external dispersant. For example, U.S. Pat. No. 5,851,280 discloses methods for the attachment of organic groups onto pigments including, for example, attachment via a diazonium reaction wherein the organic group is part of the diazonium salt. The resulting surface-modified pigments can be used in a variety of applications, such as inks, inkjet inks, coatings, toners, plastics, rubbers, and the like. These modified pigments provide inkjet inks with good overall properties.
Polymers other than dispersants have also been incorporated into inkjet ink compositions in an effort to improve performance. For example, emulsion polymers or latexes have been used as an additive to improve the print performance of inkjet inks. Since an emulsion polymer typically has a high molecular weight, this enables the use of higher molecular weight materials in an inkjet ink application. However, as emulsion polymers, these polymers are not soluble and would tend to lead to difficulties in printing, particularly nozzle clogging.
Other polymer-based inks are also known. For example, U.S. Pat. No. 6,281,267 describes an inkjet ink set comprising a first ink comprising a pH-sensitive polymer and a self-dispersing pigment and a second ink. When the first ink comes in contact with the second ink on a print medium, the polymer becomes insoluble and precipitates onto the print medium. This ink set is believed to have improved bleed and halo control.
Thus, as the inkjet printing industry moves towards print performance similar to that of laser printing, there remains a need for additional inkjet ink compositions and systems with improved print properties, such as improved print durability.